Digitalization of television (TV) broadcasting has been actively promoted in recent years by many countries to reduce waste of channel resources and to improve transmission quality of audiovisual content. An antenna device, a pivotal component for receiving digital TV signals transmitted by wireless TV stations between an audiovisual device and a propagation medium, has to not only convert between electrical and electromagnetic domains, but to also reduce electromagnetic attenuation in a specific frequency band to improve a signal-to-noise ratio (SNR) of the signal received thereby. For current digital TV broadcasting, the frequency band usually ranges from 470 MHz to 700 MHz.
Referring to FIG. 1, a conventional antenna device includes two radiating portions 11 that are symmetrical with respect to an axis (not shown) and that each have an isosceles triangular shape, and a feed portion 12 that is arranged on the axis. Gain performance of the conventional antenna device in the abovementioned frequency band is represented in FIG. 4 by a curve 81, and has room for improvement.